I'll second that comment about charging, it ~is~ quite dangerous in terms of fire or even explosion charging a Li-Ion pack without a proper Li-Ion charger. Though if you insist on doing it, there's two critical specs.
Voltage can not exceed 4.2V per cell for a standard Li-Ion. Though there are high voltage Li-Ions that terminate charge at 4.35V per cell, they are not typically used in power tool packs. This can be problematic for a pack charger in lieu of a balance charger that does not have a mechanism to balance cells. Pack chargers have no way of preventing an imbalance that can cause a cell to go over tolerance, specifically that's 4.25V per cell absolute maximum with less being preferable.
For a standard Li-Ion, charge current can not exceed 1C or one times the charge capacity. Power tool packs use high drain cells which are typically good up to a 2C charge rate or two times the charge capacity. In your case that would be 2.6 Amps. Current limits are not something easily guaranteed by a simple power supply.
As long as you don't violate either of those two specifications, you'll be good. However, it's not something easily ensured without a proper Li-Ion charger. Without a tightly regulated supply, voltage tolerance can easily be exceeded. When packs are depleted, charge current can and likely will exceed tolerance.